Many modern electronic systems employ a volatile memory for temporary storage, caching and buffering, such as a DRAM arranged as a plurality of memory dies organized into banks of devices which process commands. The dies are coupled to a memory controller by a bi-directional memory bus. The memory bus directs commands from a memory controller to the various dies, and directs data from the dies to the memory controller.
Data is stored on DRAMs as a memory cell charge. Over time, the memory cell charge may “leak” or dissipate, causing increased occurrence of memory errors in the data stored on the DRAM. In order to prevent data loss and minimize memory errors, refresh commands need to be issued periodically at a set rate to refresh the charges storing data in the die. Memory cell charge leakage is particularly linked to increased temperatures of the memory die, and gets worse as the temperature rises. The refresh rate must be higher when the die is operating at a high temperature in order to prevent occurrence of memory errors.
In typical systems, the refresh rate needs to increase when the temperature exceeds a set threshold, in many cases a first threshold is set to 85° C. For example, refresh commands are set to occur at a first average rate at temperatures up to 85° C. degrees, and are set to double to a second average rate at temperatures from 85° C. degrees to 95° C. degrees. Adjusting the refresh command rates at a specified threshold temperature requires that the refresh rate within the range of temperatures up to that threshold accommodates the maximum temperature in the range. For example, the refresh rate for all temperatures below 85° C. must be high enough to minimize data loss at or near 85° C., resulting in far more refresh cycles at low temperatures, such as room temperature or 60° C., than is necessary. As refresh cycles consume power and occupy the memory bus during execution, this overuse of unnecessary refresh cycles at low temperatures is inefficient.
Memory devices do not provide any sensor or other means of directly measuring the temperature of the memory die. In the absence of such a direct measurement, there is a need for a method of accurate measurement of the memory die temperature by alternative means. Methods which do not require the modification of the memory device are obviously advantageous as they do not require redesign of a memory device and so may work with any existing or future memory device.
Accordingly, there is a long-felt need to correct the problems inherent to present day systems.